Trisomy occurs when there are three copies of a chromosome instead of two (all chromosomes usually go in pairs). Although most prospective parents are familiar with Down syndrome and will be evaluated for it, other potentially more serious trisomies can occur, such as Edwards syndrome, Patau syndrome, and others.
Some of these can cause minor symptoms, if any. Others can cause serious defects that make life unviable, or even pregnancy.
The genes that contain all the information encoded by DNA related to physiological composition and metabolic function are found on chromosomes . Each nucleus of a human cell generally contains 46 chromosomes, 23 of which we inherit from each genetic parent.
Of these, 22 pairs are autosomes that define our unique biological and physiological characteristics. The 23rd pair is the sex chromosomes (X or Y), which largely determine biological sex.
In rare cases, a coding error can occur when a cell divides during fetal development. Instead of completely dividing into two identical chromosomes, the newly divided chromosome will have additional genetic material.
This can lead to a complete trisomy (in which a complete third chromosome is created) or a partial trisomy (in which only part of the chromosome is copied). From this point on, the error will repeat and repeat as the cell continues to divide.
Down syndrome , the most common genetic disorder in humans, is called trisomy 21 because there is an extra copy of chromosome 21 in the nucleus of each cell. Other genetic disorders are also named similarly .
Causes and consequences
Trisomies involving the sex chromosomes, in which genetic females generally have two X (XX) chromosomes and genetic males have X and Y (XY) chromosomes, are generally less severe. Autosomal trisomies often cause severe physical and mental impairment, especially complete autosomal trisomy, so premature death is common .
In addition to birth defects, trisomy can undermine the viability of the pregnancy. In fact, more than half of all miscarriages are believed to be directly related to a chromosomal defect. Many of them are associated with trisomies .
No one knows exactly why chromosome 21 is so vulnerable to trisomy. Of all the trisomies that researchers have identified, Down syndrome is known to affect nearly one in 800 births worldwide. These trisomies are much less common, but they are worth knowing about.
Edwards syndrome (trisomy 18)
Edwards syndrome (trisomy 18) is rare, occurring in only one in 5,000 births. About 95% of cases are caused by an extra chromosome 18. The remaining 5% of cases are due to an error known as a translocation, in which the building blocks of one chromosome are inserted into another .
Edwards syndrome is characterized by low birth weight, an abnormally small head, and defects in the heart, kidneys, lungs, and other organs. Although some children with Edwards syndrome reach adolescence, most die within the first year (and often the first few days) of life .
Patau syndrome (trisomy 13)
Patau syndrome (trisomy 13) is the third most common autosomal disorder in newborns after Down syndrome and Edwards syndrome. Most cases are associated with complete trisomy; very little is caused by translocation or a similar condition known as mosaicism, in which the building blocks of a chromosome are rearranged .
Children with Patau syndrome often have a cleft lip and palate, extra fingers or toes, heart defects, severe brain abnormalities, and deformed or crooked internal organs. The severity of the symptoms is such that a child with Patau syndrome rarely reaches the first month.
Varkani syndrome (trisomy 8)
Varkani syndrome (trisomy 8) is a common cause of miscarriage and usually results in the death of the newborn within the first few months. Babies born with Varkani syndrome usually have a cleft palate, characteristic facial features, heart defects, joint defects, abnormal or missing kneecaps, and an abnormally curved spine (scoliosis) .
Trisomy 16 is the most common autosomal trisomy seen in miscarriages and accounts for at least 15% of first trimester pregnancy losses. Complete trisomy 16 is incompatible with life. While most fetuses with this abnormality are spontaneously aborted by the 12th week of pregnancy, some survive until the second trimester .
In contrast, the survival chances of children with mosaic trisomy 16 were previously considered negligible, as most deaths occurred in early childhood.
Since then, advances in genetic research have shown that some previously unidentified children with mosaic trisomy 16 do not have any abnormalities and that the risk of miscarriage and birth defects is directly related to the number of cells that carry the chromosome mutation. .
Additionally, more than half of children with mosaic trisomy 16 will have fetal abnormalities, including defects in the musculoskeletal system, distinctive facial features, small lungs, and a defect of the atrial septum (a hole between the upper chambers of the heart).
Men often have hypospadias , in which the opening of the urethra develops on the shaft of the penis rather than at the end. Developmental delays are possible, but they are less common than other trisomies .
Trisomy 22 it is the second most common chromosomal cause of miscarriage. Children with complete trisomy 22 rarely survive the first trimester. The severity of physical and organic defects is such that full-term babies cannot survive for more than a few hours or days .
Some children with mosaic trisomy 22 survive. The severity of birth defects is determined by the number of cells with a mutated chromosome copy. Screening signs include heart abnormalities, kidney problems, mental retardation, muscle weakness, and cognitive decline and developmental delays .
Trisomy 9 is a rare condition in which complete trisomy is usually fatal within the first 21 days of life. Newborns with trisomy 9 will have a smaller head, distinctive facial features (including a bulging nose and sloping forehead), a deformed heart, kidney problems, and often severe muscle and skeletal deformities .
Babies born with partial or mosaic trisomy 9 are much more likely to survive. This is especially true with mosaic trisomy 9, in which organ defects tend to be less severe and intellectual disabilities do not necessarily interfere with basic language, communication, or social-emotional development.
Since the disease was first identified in 1973, several cases of mosaic trisomy 9 have been identified in the medical literature .
Klinefelter syndrome (XXY syndrome)
Klinefelter syndrome , also known as XXY syndrome, is a disease that affects men and is caused by an extra X chromosome. People with Klinefelter syndrome often produce low levels of testosterone, which leads to a decrease in muscle mass, facial and body hair.
Typical symptoms include small testicles, delayed development, enlarged breasts (gynecomastia), and decreased fertility. The severity of symptoms can vary greatly.
Some people with Klinefelter syndrome may also have learning disabilities, which are often language-oriented, although intelligence is usually normal. Testosterone replacement therapy is often used to treat the disorder in conjunction with fertility support treatments for those seeking to have children .
Triple X syndrome (trisomy X)
Some women are born with triple X syndrome, which includes an extra X chromosome. Triple X syndrome, also known as XXX syndrome, is not physically related and often does not cause any medical symptoms.
A small proportion of those affected may have menstrual irregularities, as well as learning disabilities, speech delays, and language disorders. However, most of them will develop normally and without hindrance .
Most men born with an extra Y chromosome do not have physical characteristics or health problems. If anything, people with XYY syndrome can sometimes be above average and may be at increased risk for learning disabilities, as well as delays in speech and language skills.
The disorder, if any, tends to be mild. Most adults with XYY syndrome have normal sexual development and are able to conceive .